Can You Run a Furnace on a Generator?

A gas furnace can run on a generator, and in a climate where a long outage in January is a genuine hazard it is a reasonable thing to plan for. The furnace itself is a small load — an inducer motor, a blower, a control board and an ignitor — and the part that gets people into trouble is never the furnace. It is how the generator gets connected to the house, and where the generator sits while it runs. A house that loses heat without a plan is an emergency no-heat call that has to compete with every other one in the county on the same night.

Why does a generator need a transfer switch?

A transfer switch or a generator interlock exists to make it physically impossible for generator power and utility power to be present on the same wiring at the same time. Without that, power from your generator travels back out through your service and onto the utility line, where the distribution transformer steps it back up to line voltage. Crews working to restore your neighborhood have no way to know it is there.

The improvised alternative — a double-ended cord into a dryer outlet — also leaves exposed energized prongs at whichever end is not plugged in, bypasses the circuit protection that was designed into your panel, and gives you no way to control which circuits are live. There is a reason electricians have a grim nickname for that cord. A properly installed interlock kit on the panel, or a small manual transfer switch feeding the furnace, well pump and a few outlets, is not an expensive piece of work and it is the entire difference between a safe setup and a dangerous one.

This is real for people out past the end of the line. Restoration in rural Douglas and St. Louis County can take longer than in town, and households around Hawthorne and Highland often plan on being on their own for a while after a heavy wet-snow event.

Where does carbon monoxide come from in an outage?

The carbon monoxide risk during an outage comes from the generator far more often than from the furnace. A portable generator's engine exhaust contains CO at concentrations that can be lethal quickly in an enclosed or partly enclosed space, and people put them in garages, on porches and inside breezeways to keep the snow off them. An open garage door does not make a garage safe.

• Run the generator outdoors, on a clear surface, as far from the house as the cord allows.
• Point the exhaust away from the building and keep it away from windows, doors, fresh air intakes and dryer vents.
• Do not run one under a deck, in a shed, in an attached garage or in a breezeway.
• Keep the area around it shoveled so exhaust cannot pool in a snow trench and drift back toward the house.
• Shut it down and let it cool before refueling, and store fuel outside the living space.
• Have working CO alarms on every level, tested, with fresh batteries before the season starts.

If an alarm sounds, or anyone develops a headache, nausea or dizziness that improves outdoors, get everyone out to fresh air and call 911 from outside. Do not go back in to look for the source. Carbon monoxide alarms and your heating system covers placement and what the different alarm patterns mean.

Will the furnace actually run on generator power?

Most furnaces will run on a decent generator, but some control boards are fussy about power quality, and the failure is usually confusing rather than obvious. Two things matter. First, waveform and voltage regulation: an inverter generator produces power closer to what the utility supplies, while a basic contractor-grade unit can wander in voltage and frequency, which variable-speed ECM blower motors and modern boards tolerate less well than an old belt-drive furnace did.

Second, grounding and polarity. A gas furnace proves flame by flame rectification — it passes a tiny current from the flame sensor through the flame to ground — and that only works if the furnace has correct polarity and a solid ground reference. On a generator with a floating neutral, or a hookup with hot and neutral reversed, the furnace can light and then shut down a few seconds later, every time, exactly as though the flame sensor were dirty. This is a common and genuinely baffling outage problem, and it is one more reason the connection is set up by an electrician rather than improvised in the dark.

What tends to run and what decides generator size

Load	Notes
Gas furnace or boiler	Small load; usually not the deciding factor
Circulator pumps and zone valves	Small; a boiler system needs these as well as the boiler
Well pump	Large starting surge — usually what sets the generator size
Refrigerator and freezer	Cycling load, easy to stagger
Electric water heater	Large; most people do without during an outage
Sump pump	Worth including where a basement floods

Sizing is a job for an electrician, working from the actual nameplate data of what you want to run. We can tell you what the heating equipment needs; the panel and the generator are their trade.

Boilers, fuel oil and propane in an outage

A hydronic system needs its circulators as well as its boiler, and that changes the outage picture a little. A gas or oil boiler with no power does nothing, and neither do the zone valves, so a house with radiators loses heat as surely as one with a furnace — though the thermal mass of a cast-iron system means the house coasts longer before it gets uncomfortable. If you have a boiler, make sure the circuit that feeds the boiler and the circulators is on the transfer switch, not just the boiler.

Fuel oil systems have a burner motor and an ignition transformer, and some older ones are sensitive to voltage sag on startup. Annual care for fuel oil heat covers what those systems need generally. Propane systems have their own cold-weather wrinkle that has nothing to do with electricity: regulators and snow do not mix, and propane regulators in deep cold explains what to keep clear.

What to do when the power goes out in deep cold

The order of operations in an outage matters, because the first hour is when you decide whether the house holds its heat. Work through this:

1. Turn the thermostat down or off before starting the generator, so the furnace does not try to start on the initial load.
2. Set up and start the generator outdoors, then transfer the house with the switch or interlock.
3. Bring loads on one at a time, heating first, and let each one settle before adding the next.
4. Keep exterior doors closed and pull shades at night; the house loses heat fast at a −17°F to −21°F design temperature.
5. Open cabinet doors under sinks on exterior walls so room air reaches supply piping.
6. Check the furnace's outdoor vent terminations for drifting snow, since wind usually comes with the outage.
7. If the furnace will not run on generator power, stop troubleshooting it in the dark and protect the plumbing instead.

Houses in Twig and similar areas at the end of a long feeder often see the longest outages, and a plan written down before November is worth more than a plan made at midnight with a flashlight.

When to call

Call an electrician for the transfer switch, the interlock, the generator sizing and anything inside the panel. Call us for the heating equipment: a furnace or boiler that will not run on generator power, a system that locks out repeatedly during an outage, a unit that was running when the power dropped and has not been right since, or a house with no heat and no generator at all. Our 24/7 emergency heating service starts at the $89 diagnostic, credited toward the repair when you approve the work. If a CO alarm is sounding, that call goes to 911 from outside the building first.